Ruffalo has become an outspoken opponent of hydrofracking, a controversial method of extracting petroleum and natural gas from the earth used near his home in upstate New York.

"We gotta end fracking," he told me at The Avengers premiere. "Water is our most precious commodity to all life and there's a finite amount of it. And this process, along with tar sands extraction and mountain top removal, destroys water."

Not only has Ruffalo started an anti-fracking nonprofit, Water Defense, but he's also traveled to several states to attend protests and other demonstrations. Most recently, he talked about his eco-activism on The Colbert Report.

"We don't need it," Ruffalo said. "We have the technology to move past it and it's time to do it and we can do it. And we will do it."